Examples of conventional base precursors include ureas as described in U.S. Pat. No. 2,732,299 and Belgian Pat. No. 625,554, a urea or an ammonium salt of urea with weak acid as described in Japanese Patent Publication No. 1699/65, hexamethylenetetramines and semicarbazides as described in U.S. Pat. No. 3,157,503, triazine compounds and carboxylic acids as described in U.S. Pat. No. 3,493,374, dicyandiamide derivatives as described in U.S. Pat. No. 3,271,155, N-sulfonylureas as decribed in U.S. Pat. No. 3,420,665, amineimides as described in Research Disclosure, No. 15776 (1977), and thermo-degradative acid salts represented by trichloroacetic acid as described in British Pat. No. 998,949.
However, an image forming material using these base precursors has essentially serious defects. Specifically, a high image density cannot be obtained or the S/N ratio of the image is considerably decreased due to the release of base during preservation. This takes place because they do not possess good stability at normal temperature and do not smoothly decompose during the developing process.